1. Field of the Invention
The invention relates to the field of metallurgic process technology and is directed to a method for producing a magnetic alloy powder for hard-magnetic applications. The powder is formed of a samarium-cobalt base alloy. The powder can be used to produce highly coercive permanent magnets by means of hot compacting or plastic bonding. However, permanent magnets of this type can also be generated with the powder through powder metallurgy by means of sintering.
2. Description of the Related Art
Permanent magnets based on Smxe2x80x94Co were formerly produced predominantly through powder metallurgy by sintering (K. Strnat and R. M. W. Strnat, J. Magn. Magn. Mater. 100 (1991) 38). To produce the Smxe2x80x94Co powder needed for this, it is already known first to melt a corresponding alloy, to comminute this alloy after solidification, and to subject it to heat treatment in a passivation gas below the phase transformation temperature of the alloy (U.S. Pat. No. 5,122,203). A production method of this kind has the disadvantage that an energy-consuming and time-consuming multiple-stage heat treatment is needed to adjust high coercive field strengths. Further, a production method of this kind has the disadvantage that additives such as Cu and Zr are needed for Sm2Co17-type magnets in order to adjust the microstructure which enables a high coercive field strength by means of the pinning process. However, these additives reduce the saturation magnetization.
The HD process (hydride decrepitation) has long been known (U.S. Pat. No. 5,580,396, column 8, lines 30 to 41; Rare-earth Iron Permanent Magnets, ed. J. M. D. Coey, Oxford 1996, pages 346 to 349 and pages 370 to 380) in the field of magnetic powder production based on alloys with elements from the group of rare earths (RE). This process is used for the comminution of coarse, compact alloy bodies and is accordingly used to generate powders. In this connection, use is made of the effect whereby the hydrogen diffused in the intermediate grain phase or on the interstitial lattice sites of the RE compound leads to an expansion or lattice elongation of the RE compound. The tensions brought about by the expansion or lattice elongation lead to intergranular crack formation and, ultimately, to an actual bursting or pulverizing (decrepitation) of the hydrogenated material. This pulverization process can also be reinforced by the effect of vibrations (DE 28 16 538) or by the use of a swing mill (CH 560 955).
When applying the HD process for a bond Ax By, where A is a rare earth element and B represents one or more other elements (usually transition metals), the following reaction takes place:
AxBy+z/2H2xe2x86x92AxByHzxe2x80x83xe2x80x83(HD process).
Often, after the actual HD process, a removal/desorption of the hydrogen takes place during the further processing of the generated powder into the end product in the course of the subsequent process steps, for example, during sintering, wherein the following reaction takes place: AxByHzxe2x86x92H AxBy+z/2 H2.
It is also already known (EP 0 304 054; EP 0 516 264; DE 196 07 747) to apply the HDDR method (hydrogenation-disproportionation-desorption-recombination) in the production of magnetic powders in particular from Ndxe2x80x94Fexe2x80x94B alloys for improving the magnetic characteristics. In this treatment, the powder is hydrogenated in a first process step in a hydrogen atmosphere with a low pressure in the range of 0.8xc3x97105 Pa to a maximum of 0.15 MPa. As a consequence of this hydrogen treatment, a chemical reaction (disproportionation) takes place, that is, the original phase decomposes accompanied by the formation of a binary hydride and the rest of the elements or combinations of elements of the initial phase.
This chemical reaction can be shown schematically as follows (by analogous use of the above-mentioned model substance AxBy):
AxBy+z/2H2xe2x86x92AxHz+yBxe2x80x83xe2x80x83(HDDR step 1).
Subsequently, in a second process step by means of a heat treatment under vacuum conditions, the hydrogenated alloying elements are dehydrogenated again accompanied by simultaneous recombination of the alloying composition decomposed in step 1 according to the following reaction equation:
xe2x80x83AxHz+yBxe2x86x92AxBy+z/2H2xe2x80x83xe2x80x83(HDDR step 2).
A crystallite size is achieved in the range of a single-domain particle size, e.g., approximately 300 nm for Nd2Fe14B and Sm2Fe17N3, by the HDDR treatment. This grain refining, which leads to an improvement of the magnetic properties of the magnetic powder, is the main goal of the HDDR treatment and not asxe2x80x94in the HD processxe2x80x94the production of powder. It is expressly noted in this respect that the HD process is not identical to the first step of the HDDR treatment as might possibly be suggested by the first two letters of the abbreviation xe2x80x9cHDDRxe2x80x9d.
In the first step of the HDDR process, when heating up to the temperatures of 500xc2x0 C. to 10000xc2x0 C. needed for the above-mentioned reaction, the hydrogen absorption typical for the HD process, as was described above in the equation for the HD process, often comes about, but this only represents an intermediate reaction which immediately follows the desorption of the hydrogen. The HDDR treatment can be carried out in complete independence from the HD process as was shown, for example, in the xe2x80x9csolid HDDR processxe2x80x9d in which the hydrogen gas is first admitted to the reactor at the temperature needed for disproportionation (HDDR step 1); thus, no interstitial absorption of the hydrogen takes place and, accordingly, the HD process does not come about (Gutfleisch et al., J. Alloys Compd. 215 (1994) 227).
Increasing stabilization of RE-Fe compounds in the case of substitution of the Fe by Co is also known (A. Fujita and I. R. Harris, IEEE Trans. Magn. 30 (1994) 860).
It is not possible to transfer the HDDR process conditions known for Ndxe2x80x94Fexe2x80x94B magnetic powder to Smxe2x80x94Co magnetic powder because a disproportionation reaction such as that taking place under the usual HDDR conditions (500 less than T less than 1000xc2x0 C., xcx9c0.1 MPa hydrogen pressure) does not occur in the case of Smxe2x80x94Co magnetic powder because of the great stability of these alloys.
The primary object of the invention is to provide a method enabling a technologically controllable and economical production of a hard-magnetic powder composed of a samarium-cobalt base alloy for highly coercive permanent magnets.
This object is met, according to the invention, by a method enabling a technologically controllable and economical production of a hard-magnetic powder composed of a samarium-cobalt base alloy for highly coercive permanent magnets. The method is based on a HDDR treatment in which a starting powder is subjected to hydrogenation with disproportionation of the alloy in a first method step under hydrogen and, in a subsequent, second method step under vacuum conditions, a hydrogen desorption with recombination of the alloy. A starting powder containing samarium and cobalt is treated in the first method step either at a high temperature in the range of 500xc2x0 C. to 900xc2x0 C. and with a high hydrogen pressure of  greater than 0.5 MPa or by applying an intensive fine grinding at a low temperature in the range of 50xc2x0 C. to 500xc2x0 C. and with a hydrogen pressure of  greater than 0.15 MPa. By the method of the invention, magnetic alloy powders can be produced from samarium-cobalt base alloys; highly coercive permanent magnets can be produced from these magnetic alloy powders, particularly by hot compacting or plastic bonding.
Both variants of the method lead to the disproportionation of the initial phase and to the formation of a crystalline binary samarium hydride.
When applying the high temperature in the range of 500xc2x0 C. to 900xc2x0 C., a hydrogen pressure in the range of 1.0 MPa to 5.0 MPa is preferably applied.
According to an advisable configuration of the method, the intensive fine grinding is carried out for a period of 1 h to 100 h.
According to the invention, a powder of a Smxe2x80x94Co base alloy or a powder mixture comprising the individual elements of a Smxe2x80x94Co base alloy and/or comprising one or more precursor alloys suitable for the production of a Smxe2x80x94Co base alloy can be used as a starting powder when applying intensive fine grinding.
When using intensive fine grinding, the starting powder should be ground fine preferably with hydrogen pressure in the range of 0.5 MPa to 2.5 MPa.
The hydrogen desorption treatment is advisably carried out in the obtained magnetic powder by heat treatment in the range of 500xc2x0 C. to 1000xc2x0 C.
According to the invention, a starting powder which leads to magnetic alloy powders with the alloy composition SmxCo100xe2x88x92x, where 10 less than xc3x97 less than 30, or the alloy composition SmxCo100xe2x88x92xxe2x88x92axe2x88x92bxe2x88x92cFeaCubZrc, where 10 less than xc3x97 less than 30, a  less than 45, b  less than 15 and c less than 15, is preferably used.
The method according to the invention provides a new possibility for magnetic hardening of Smxe2x80x94Co base compounds. The method results in novel approaches for optimizing the magnetic properties of Smxe2x80x94Co magnets resulting in improved properties and represents an economical alternative for the production of such magnets. This includes the possibility of homogenizing the microstructure of the Smxe2x80x94Co base compounds, so that a cumbersome homogenizing at high temperatures can be dispensed with.